(1) Field of the Invention:
The present invention relates to a hydraulic fluid composition, and more particularly to a hydraulic fluid composition for automobile.
(2) Description of the Prior Art:
Central hydraulic system for automobile has been developed in order to satisfy the requirements demanded to the safe and high speed running of automobile. As the specifications for hydraulic fluids used in the central hydraulic system, SAE 71R1 (for mineral oil base hydraulic fluid) and SAE 71R2 (for synthetic oil base hydraulic fluid) are enacted in U.S.A.
In this central hydraulic system, one hydraulic fluid is used as a multipurpose hydraulic fluid for brake, power steering, automatic transmission, shock absorber, windshield wiper, seat actuator, window actuator and the like. Therefore, it is necessary that this hydraulic fluid satisfys various demands.
The synthetic base fluid for central system hydraulic fluid is demanded to have the following properties. That is, the fluid (a) has a high viscosity index, (b) is fluidable at low temperature, (c) has a high boiling point and a high flash point, (d) is excellent in the shear stability, (e) does not swell sealing material (rubber), (f) is excellent in the lubricating property, and (e) is stable against oxidation.
The hydraulic fluid composition of the present invention satisfys all the SEA 71R2 specifications as a central system hydraulic fluid, and further can be used as a hydraulic fluid for each of the above described purposes. Particularly, the hydraulic fluid composition of the present invention satisfys all the DOT-4 specifications as a brake fluid.
Polyoxyalkylene series hydraulic fluids for automobile are disclosed in U.S. Pat. No. 3,957,667 and Japanese Patent Application Publication No. 12,340/77. However, the hydraulic fluid composition disclosed in the U.S. patent is insufficient in the wet equilibrium reflux boiling point, and that disclosed in the Japanese patent application publication is insufficient in the viscosity characteristics, and therefore both the hydraulic fluid compositions cannot satisfy both the SAE 71R2 and the DOT-4 specifications.